There is no one best supplement for cyclists
cyclists, however, tart cherry juice, turmeric, caffeine, creatine, sodium bicarbonate, beta-alanine, and magnesium, have been shown to be helpful for enhancing physical performance, improving energy, and/or promoting recovery in endurance athletes.
Here’s a quick summary of the research on each:
Tart cherry juice is rich in polyphenols and antioxidants. Some evidence suggests that drinking tart cherry juice before endurance activities may improve performance and reduce muscle soreness, although research is mixed .
Turmeric is a potent antioxidant with anti-inflammatory properties. Small studies indicate that 150-1500mg turmeric/day may accelerate post-exercise recovery and reduce muscle soreness. However, the evidence is mixed 
Caffeine: Taking in caffeine prior to exercise may result in improved performance, speed, power, and endurance capacity 
. Caffeine is a well-known central nervous system stimulant, found in coffee, tea, and chocolate.
Creatine: Creatine supplementation may improve muscular strength, body composition, and speed and enhance recovery 
Sodium bicarbonate: Small studies suggest that sodium bicarbonate may reduce fatigue and improve performance during high-intensity activities shorter than 4 minutes. That said, it does not seem to have an impact on endurance activities like cycling .
Beta-alanine: Beta-alanine may help improve performance and reduce neuromuscular fatigue, although the direct impact on endurance activities is unclear 
Magnesium: Studies suggest that magnesium supplementation may aid in sleep and relaxation, both of which are important for recovery 
 Ruirui Gao & Philip D. Chilibeck (2020) Effect of Tart Cherry Concentrate on Endurance Exercise Performance: A Meta-analysis, Journal of the American College of Nutrition, 39:7, 657-664, DOI: 10.1080/07315724.2020.1713246
 Curcumin supplementation and delayed onset muscle soreness (DOMS): effects, mechanisms, and practical considerations Phys Act Nutr. 2020;24(3):39-43. Published online September 30, 2020, DOI: https://doi.org/10.20463/pan.2020.0020
 Jonathan D. Wiles, Damian Coleman, Michael Tegerdine & Ian L. Swaine (2006) The effects of caffeine ingestion on performance time, speed and power during a laboratory-based 1 km cycling time-trial, Journal of Sports Sciences, 24:11, 1165-1171, DOI: 10.1080/02640410500457687
 Rawson, E. S., Miles, M. P., & Larson-Meyer, D. E. (2018). Dietary Supplements for Health, Adaptation, and Recovery in Athletes, International Journal of Sport Nutrition and Exercise Metabolism, 28(2), 188-199. Retrieved Jun 18, 2021, from http://journals.humankinetics.com/view/journals/ijsnem/28/2/article-p188.xml
 McNaughton, Lars R.; Siegler, Jason; Midgley, Adrian Ergogenic Effects of Sodium Bicarbonate, Current Sports Medicine Reports: July 2008 - Volume 7 - Issue 4 - p 230-236, doi: 10.1249/JSR.0b013e31817ef530
 Trexler, E.T., Smith-Ryan, A.E., Stout, J.R. et al. International society of sports nutrition position stand: Beta-Alanine. J Int Soc Sports Nutr 12, 30 (2015). https://doi.org/10.1186/s12970-015-0090-y
 Henry C Lukaski, Magnesium, zinc, and chromium nutriture and physical activity, The American Journal of Clinical Nutrition, Volume 72, Issue 2, August 2000, Pages 585S–593S, https://doi.org/10.1093/ajcn/72.2.585S